1. Field of the Invention
The present invention relates to a glass substrate and thin film combination used for, for example, a liquid crystal display device, and a method for producing the same.
2. Description of the Related Art
Recently, liquid crystal panels have been increasingly demanded. In many liquid crystal display devices including a liquid crystal panel, a glass substrate is coated with a thin film formed of a metal, semiconductor, or insulative material directly deposited thereon to be used as a material for an electrode or the like. In liquid crystal display devices including transistors or MIM devices, the glass substrate needs to be sufficiently flat. A glass substrate which is flat and also smooth is conventionally used. Such a smooth glass substrate has inferior adhesion with a thin film formed of a metal or the like and thus causes the thin film to peel off relatively easily.
With reference to FIG. 10, a cause of the peeling-off will be described. FIG. 10 is a cross sectional view of a conventional glass substrate and thin film combination 30 including a smooth glass substrate 1 and a thin film 2 provided thereon.
The thin film 2 is directly deposited on the smooth glass substrate 1 having irregularities, each of which has a height of about 1 nm to about 5 nm and a width of about 100 nm to about 2 .mu.m. In this specification, the term "width" refers to a distance between two adjacent irregularities. A surface having irregularities each having a height of about 5 nm or less and a width of about 2 .mu.m or more is defined as a "smooth surface".
A stress generated by thermal strain and by the glass substrate 1 and the thin film 2 which is applied during the deposition of the thin film 2 concentrates on the interface between the glass substrate 1 and the thin film 2 as an inner stress 4. As a result, a strain is generated at a portion 5 at which the adhesion between the glass substrate 1 and the thin film 2 is weaker than the rest of the interface. Such a strain causes the peeling-off. A thin film formed of a low-resistance metal such as Al, Ta, Mo, Mo--Ta, or Ti peels off more significantly. A semiconductor thin film formed of a single element such as Si or Ge or a mixed crystal such as GaAs or Si--Ge, and an insulative thin film formed of a nitride such as SiN.sub.x or an oxide such as SiO.sub.x also peel off relatively easily by the same mechanism.
Conventionally, in order to avoid such peeling-off, an insulative thin film is formed on the glass substrate by spin-coating, CVD or PVD as an overcoat before the thin film is deposited. The overcoat is used in order to provide protection of the surface of the glass substrate which is required by the process and also in order to avoid concentration of the stress in a local area in the thin film. The overcoat, which has a quite rough surface morphology, alleviates the stress and thus maintains the adhesion between the glass substrate and the thin film at a satisfactory level. However, formation of such an overcoat increases the number of production steps of the display device and thus raises production cost.